Subtalar Arthroereisis: Biomechanics, Surgical Techniques, and Clinical Outcomes

INTRODUCTION TO SUBTALAR ARTHROEREISIS

The term arthroereisis is derived from the Greek roots arthro- (joint) and -ereisis (to prop up or support). In orthopedic surgery, subtalar arthroereisis refers to the surgical insertion of an implant into the sinus tarsi to limit excessive subtalar joint pronation while preserving physiological range of motion.

The conceptual foundation of this procedure dates back to Grice’s description of an extra-articular arthrodesis, which involved the insertion of a bone graft into the sinus tarsi. Grice’s original premise was that by mechanically limiting the ability of the calcaneus to externally rotate and the talus to internally rotate, the longitudinal arch of the foot could be restored and maintained. Over the decades, this concept has evolved significantly. Modern subtalar arthroereisis has transitioned away from arthrodesis (fusion) toward dynamic, joint-sparing stabilization using various synthetic implants. These devices are strategically placed in the sinus tarsi to block the external rotation of the calcaneus relative to the talus, or to prevent the anterior process of the calcaneus from abutting the lateral process of the talus during the pronation phase of the gait cycle.

While the procedure has gained widespread acceptance in the management of pediatric flexible flatfoot and neuromuscular deformities, its application in adult acquired flatfoot deformity (AAFD) remains a subject of rigorous academic debate. Proper patient selection, precise surgical technique, and a profound understanding of hindfoot biomechanics are absolute prerequisites for achieving satisfactory clinical outcomes.

BIOMECHANICS AND IMPLANT CLASSIFICATION

The subtalar joint is a complex, single-axis joint that functions analogously to a mitered hinge, translating transverse plane rotation of the tibia into frontal and sagittal plane motion of the foot. In the pathologically pronated foot, the talus plantarflexes and adducts, while the calcaneus everts. Subtalar arthroereisis implants function by physically obstructing this excessive motion.

As comprehensively described by Maxwell and Cerniglia, sinus tarsi implants can be categorized into three distinct biomechanical types:

1. The Self-Locking Wedge

The self-locking wedge is the most commonly utilized design in contemporary practice. It is inserted in a screw-like fashion directly into the sinus tarsi, wedging between the lateral process of the talus and the anterior process of the calcaneus.
* Mechanism: It acts as an extra-articular mechanical block that prevents the anterior process of the calcaneus from sliding forward and outward, thereby halting excessive external rotation of the calcaneus on the talus.
* Examples: Various titanium and polyetheretherketone (PEEK) threaded cylinders.

2. The Axis-Altering Device

Unlike the self-locking wedge, the axis-altering device is an intra-articular implant.
* Mechanism: It is inserted deep under the lateral process of the talus, residing in the lateral-most portion of the posterior subtalar joint facet. By physically elevating the lateral aspect of the talus, it alters the spatial orientation of the subtalar joint axis (Henke's axis), thereby shifting the kinematics of the hindfoot to favor supination over pronation.
* Examples: The STAY-peg implant (Wright Medical, Arlington, TN), typically manufactured from ultra-high-molecular-weight polyethylene (UHMWPE).

3. The Impact-Blocking Device

The impact-blocking device requires direct osseous integration.
* Mechanism: It is surgically impacted or screwed directly into the bony floor of the sinus tarsi (the calcaneal sulcus). A superiorly projecting peg or abutment acts similarly to the self-locking wedge by physically blocking the lateral process of the talus as it attempts to internally rotate and plantarflex during pronation.
* Examples: Various stemmed titanium or bioabsorbable pegs.

Clinical Pearl: The choice of implant material significantly impacts long-term outcomes. While titanium devices offer superior structural integrity, they may induce localized osteolysis or require removal due to sinus tarsi pain. Bioabsorbable implants (e.g., Giannini polylactic acid devices) theoretically eliminate the need for secondary removal but have been associated with sterile inflammatory responses, premature fragmentation, and loss of correction, particularly in adult patients.

INDICATIONS AND PATIENT SELECTION

The literature consistently indicates that the most predictable and successful outcomes of subtalar arthroereisis are achieved in specific patient populations.

Pediatric Flexible Pes Planovalgus

The primary indication for subtalar arthroereisis is symptomatic, flexible pes planovalgus in pediatric patients (typically aged 8 to 15 years) who have failed exhaustive conservative management (e.g., custom orthoses, physical therapy, activity modification). In this demographic, the implant not only corrects the immediate deformity but may also guide the remodeling of the tarsal bones during skeletal maturation.

Neuromuscular Flatfoot Deformity

Children with neuromuscular disorders, such as cerebral palsy or Down syndrome, frequently develop severe, progressive planovalgus deformities due to muscle spasticity or ligamentous laxity. Arthroereisis, often combined with soft tissue balancing (e.g., Achilles tendon lengthening, medial plication), provides a robust, joint-sparing alternative to lateral column lengthening or early arthrodesis.

Adult Acquired Flatfoot Deformity (AAFD)

The use of arthroereisis in adults is highly controversial. It is generally reserved as an adjunct procedure in Stage IIb AAFD, performed concomitantly with flexor digitorum longus (FDL) transfer, medializing calcaneal osteotomy (MCO), and spring ligament reconstruction. It is rarely, if ever, indicated as an isolated procedure in the adult population due to the high incidence of postoperative sinus tarsi pain and implant failure.

Contraindications

• Rigid Deformity: Tarsal coalitions or rigid arthritic flatfeet will not benefit from arthroereisis and require arthrodesis.
• Advanced Degenerative Joint Disease: Pre-existing subtalar or talonavicular osteoarthritis.
• Severe Frontal Plane Deformity: Forefoot varus exceeding 15 degrees that cannot be corrected with a medial column procedure.
• Morbid Obesity: Significantly increases the risk of implant extrusion or fragmentation.

PREOPERATIVE EVALUATION

A meticulous preoperative assessment is mandatory.

Clinical Examination

• Silfverskiöld Test: Assess for isolated gastrocnemius tightness versus global Achilles contracture. A tight Achilles complex will drive the midfoot into a rocker-bottom deformity if not addressed concomitantly.
• Hubscher Maneuver (Jack's Test): Passive dorsiflexion of the hallux should reconstitute the medial longitudinal arch, confirming the flexibility of the deformity.
• Heel Rise Test: Evaluates posterior tibial tendon function and hindfoot flexibility.

Radiographic Assessment

Standard weight-bearing anteroposterior (AP) and lateral radiographs of the foot, along with a hindfoot alignment view, are required.
* Lateral View: Assess Meary’s angle (talo-first metatarsal angle), calcaneal pitch, and talocalcaneal angle.
* AP View: Evaluate talonavicular coverage (uncovering >30% indicates severe abduction) and the AP talocalcaneal (Kite's) angle.

SURGICAL TECHNIQUE: STEP-BY-STEP

1. Patient Positioning and Anesthesia

The patient is placed in the supine position on the operating table. A bump is placed under the ipsilateral hip to internally rotate the leg, bringing the lateral aspect of the foot into clear view. A thigh or calf tourniquet is applied. Regional anesthesia (popliteal block) combined with general anesthesia or deep sedation is recommended.

2. Incision and Dissection

• A 2 to 3 cm oblique incision is made directly over the sinus tarsi, following the relaxed skin tension lines, approximately 1 cm inferior and anterior to the tip of the lateral malleolus.
• Blunt dissection is carried out through the subcutaneous tissue.

  Surgical Warning: Extreme care must be taken to identify and protect the intermediate dorsal cutaneous nerve (superiorly) and the sural nerve (inferiorly). Neuroma formation in this region is a devastating complication.

• The extensor digitorum brevis (EDB) muscle belly is identified and retracted distally and inferiorly. In some cases, a small portion of the EDB origin may be elevated from the calcaneus to expose the sinus tarsi.

3. Preparation of the Sinus Tarsi

• The contents of the sinus tarsi, including the Hoke’s tonsil (fat pad), are partially excised to allow visualization of the tarsal canal.
• The cervical ligament and the interosseous talocalcaneal ligament are identified. While some surgeons advocate for complete preservation of these ligaments, partial release is often necessary to accommodate the implant and achieve adequate reduction of the talus.
• A Freer elevator or a specialized sinus tarsi probe is passed from lateral to medial through the tarsal canal to ensure patency. The probe should be palpable on the medial side of the foot, just inferior to the medial malleolus.

4. Trialing and Sizing

This is the most critical step of the procedure.
* A guide wire is passed through the sinus tarsi from lateral to medial.
* Progressively larger trial implants are inserted over the wire.
* With each trial, the surgeon assesses the hindfoot range of motion. The goal is to allow approximately 2 to 4 degrees of subtalar eversion.

Pitfall: Over-sizing the implant ("overstuffing" the sinus tarsi) will force the hindfoot into rigid varus, leading to lateral column overload, fifth metatarsal stress fractures, and intractable sinus tarsi pain. Under-sizing will fail to correct the deformity and may lead to implant extrusion.

5. Implant Insertion

• Once the optimal size is determined, the definitive implant is inserted. For self-locking wedges, the implant is screwed into place until the lateral edge of the device is recessed 2 to 3 mm deep to the lateral wall of the calcaneus.
• Intraoperative fluoroscopy (AP, Lateral, and Harris axial views) is utilized to confirm the precise position of the implant and the correction of radiographic parameters (e.g., restoration of Meary's angle and talonavicular coverage).

6. Concomitant Procedures

Subtalar arthroereisis is rarely a standalone procedure. Depending on the preoperative assessment, the following may be required:
* Percutaneous Achilles tendon lengthening (TAL) or gastrocnemius recession.
* Kidner procedure or posterior tibial tendon advancement.
* Medial cuneiform opening wedge osteotomy (Cotton procedure) for residual forefoot varus.

7. Closure

The EDB fascia is meticulously repaired over the sinus tarsi to provide a soft tissue buffer and prevent implant extrusion. The subcutaneous tissue and skin are closed in a standard layered fashion.

POSTOPERATIVE PROTOCOL

Postoperative management is largely dictated by the concomitant procedures performed.
* Weeks 0-2: The patient is placed in a well-padded short leg splint or cast and remains strictly non-weight-bearing. Elevation and strict rest are emphasized to minimize edema and wound complications.
* Weeks 2-6: Sutures are removed. The patient is transitioned to a controlled ankle motion (CAM) boot. Weight-bearing is gradually advanced based on radiographic evidence of healing (if osteotomies were performed) and clinical tolerance.
* Weeks 6-12: Transition to supportive athletic shoes with custom orthoses. Aggressive physical therapy is initiated, focusing on peroneal strengthening, posterior tibial tendon rehabilitation, and proprioceptive training.

CLINICAL OUTCOMES AND COMPLICATIONS

A critical review of the literature reveals conflicting outcomes, heavily dependent on patient age and underlying pathology.

Pediatric and Neuromuscular Outcomes

In the pediatric population, results are generally highly favorable. Giannini et al. (2001) reviewed the use of polylactic acid resorbable implants in children aged 8 to 15 with bilateral functional flexible pes planus, reporting significant improvements in both radiographic and clinical parameters with a remarkably low complication rate. Similarly, Vedantam et al. evaluated the STAY-peg device in 140 planovalgus feet in children with neuromuscular disorders. With concomitant soft tissue balancing, satisfactory results were reported in 96% of feet, effectively preventing the progression of deformity and delaying or eliminating the need for triple arthrodesis.

Metcalfe et al. conducted a systematic review demonstrating patient satisfaction rates ranging from 79% to 100% in pediatric cohorts. However, they noted that arthroereisis is not benign; complication rates ranged from 5% to 18%.

Adult Outcomes and Complications

The use of arthroereisis in adults is fraught with higher complication rates. The adult subtalar joint is less adaptable, and the introduction of a mechanical block frequently leads to localized impingement.
* Sinus Tarsi Pain: This is the most common complication, occurring in up to 30% of adult patients. It is often due to implant micromotion, over-sizing, or localized synovitis.
* Implant Removal: A significant incidence of pain necessitates implant removal. Notably, the literature indicates that pain does not always resolve following removal, suggesting permanent alteration or damage to the interosseous ligaments or articular cartilage.
* Implant Fragmentation: Bioabsorbable interference screws, initially thought to be advantageous as they eliminate the need for removal, have shown poor results in adults. Saxena and Nguyen reported significant collapse and fragmentation of bioabsorbable implants in adult patients, leading to severe pain and necessitating complex revision surgeries.

SUMMARY AND CURRENT CONSENSUS

Based on an exhaustive review of current orthopedic literature, several definitive conclusions regarding subtalar arthroereisis can be drawn:

1. Efficacy in Deformity Reduction: The insertion of a sinus tarsi blocking implant consistently reduces flexible pes planus deformity. In short- to mid-term follow-up, it successfully maintains this reduction, particularly when combined with appropriate soft tissue or osseous procedures.
2. Complication Profile: There is a clinically significant incidence of sinus tarsi pain that may require implant removal. Surgeons must counsel patients preoperatively that removal may be necessary and that pain may persist even after the device is explanted.
3. Lack of Long-Term Data: Current literature is characterized by mid-term follow-up. High-quality, prospective, long-term studies (10+ years) are still required to evaluate the incidence of adjacent joint arthritis.
4. Optimal Patient Population: The highest success rates and lowest complication profiles are observed in pediatric patients with symptomatic flexible pes planus and children with neuromuscular disorders. Its use in adult acquired flatfoot deformity should be approached with extreme caution and reserved for highly selected cases as an adjunct to comprehensive reconstruction.

📚 Medical References

• Subtalar arthroereisis for the correction of planovalgus foot in children with neuromuscular disorders, J Pediatr Orthop 18:294, 1998.
• Yazici M, Ahser MA, Hardacker JW: The safety and effi cacy of Isola-Galveston instrumentation and arthrodesis in the treatment of neuromuscular spinal deformities, J Bone Joint Surg 82A:524, 2000.
• Muscular Dystrophy—General Bailey RO, Marzulo DC, Hans MB: Muscular dystrophy: infantile fascioscapulohumeral muscular dystrophy—new observations, Acta Neurol Scand 74:51, 1986.
• Becker PE: Two new families of benign sex-linked recessive muscular dystrophy, Rev Can Biol 21:551, 1962.
• Berman AT, Garbarino JL, Rosenberg H, et al: Muscle biopsy: proper surgical technique, Clin Orthop Relat Res 198:240, 1985.
• Bowen TR, Miller F, Mackenzie W: Comparison of oxygen consumption measurements in children with cerebral palsy to children with muscular dystrophy, J Pediatr Orthop 19:133, 1999.
• Brown RH, Phil D: Dystrophy-associated proteins and the muscular dystrophies, Annu Rev Med 48:457, 1997.
• Copeland SA, Levy O, Warner GC, et al: The shoulder in patients with muscular dystrophy, Clin Orthop Relat Res 368:80, 1999.
• Gardner-Medwin D: Clinical features and classifi cation of muscular dystrophies, Br Med Bull 36:109, 1980.
• Green NE: The orthopaedic care of children with muscular dystrophy, Instr Course Lect 36:267, 1987.
• Hsu JD, Hoffer MM: